Malcolm Sinclair wants power. Elected president of Equity in July, the busy actor – currently playing Benjamin Britten in Alan Bennett's The Habit of Art at the National – wants to prove to a sceptical acting profession that its union still has clout. "We've got to maximise the power we have," he says, "in order to protect Equity's membership – the performers, the stage managers, the directors, the writers. And also," he adds ominously, "to protect the industry."

Equity was once one of the most powerful of Britain's trade unions: no actor or dancer was allowed to perform without a coveted Equity card, limited numbers of which were released each year. But this "closed shop" system fell foul of employment legislation in 1988; since then, membership has been optional, meaning that the union's power has been seriously eroded.

Equity still runs successful campaigns: in 2008 it negotiated a wage increase of more than 40% for actors in West End shows, bringing the minimum weekly payment to £450. And membership is slowly on the rise (37,000 compared with 35,000 two years ago). But with a growing number of fringe theatre companies unable – or unwilling – to pay their actors, some argue that the union has lost its bite. Shelling out around £100 a year for Equity membership makes little sense when you're not even getting paid.

Sinclair smiles wearily at this comment. "That's bollocks," he says. "If that [anti-vermin campaign] was all we were doing, then we'd have to give up."

Sinclair joined Equity in the 1970s, but only became politically active four years ago, when he led a successful campaign to halt the closure of the Bristol Old Vic theatre. He believes passionately in the union, he says, because he wants to "pay his dues". "I get my holidays, my pay, my overtime. I wouldn't get any of those if it wasn't for the union."

So what about that small army of fringe performers working for little or no money: do they still owe Equity a debt of duty? Last summer, a member of the Equity council – the elected body that runs the union – suggested that all fringe companies be forced to pay their actors the minimum wage, a proposal overturned on the basis that it would spell the end of the fringe.

"Would we prefer actors to keep their skills up and perform for free," asked one actor on the blog, "or would we prefer a blanket rule that effectively kills the fringe?"

On this point, Sinclair is emphatic. "If someone is making money out of something," he says, "the actors involved have to have their share. If nobody's making money – and I know people who have taken mortgages out to put on shows, and lost the lot – then it's up to the actor. But if members come to us and say, we are being exploited, we'll be on to it like a . . ." He laughs, searching for the right expression. "Like a ferret."

Sinclair's presidency does appear to be ushering in a new era for Equity: the union's new council is the youngest in living memory, and the first to contain a majority of women. Sinclair believes it's ready to meet the demands of what he calls, with wry understatement, a "pretty tricky time for the arts".

Equity has written to David Cameron to protest against the coalition's planned arts funding cuts, though they have had no reply yet. The union's basic position is to oppose all cuts, but Sinclair admits that this might be wishful thinking. "It is unrealistic to imagine [that there won't be any cuts]. It depends what they are and how they fall."

Sinclair is also concerned about what he terms the "shrinking" of British theatre. " In London it has a good, healthy audience," he says. "But cities like Chester and Worcester used to have wonderful theatres, and now they're gone. Equity can't create work, but we can say, in order to have a healthy industry, this is where we should be concentrating our energies."

Some see theatre being eroded in more subtle ways. In an age in which TV talent show winners are catapulted into lucrative West End jobs, asked one person, how can Equity "change the image of the profession so that it is respected again?"

"Well, that's a marvellous question," Sinclair says. "Part of the acting job is about self-respect – to have enough self-belief to go out in front of 600 people for two hours and make them listen to you. And I'd say that one of the reasons to belong to Equity is to get some sort of self-respect, to realise that you're not on your own. So if you feel you're being shafted, give us a call."